1. Field of the Invention
This invention relates to friction stir welding and, more particularly, to an anvil used in friction stir welding processes that is either coated with or manufactured from a substance capable of resisting anvil deformation and preventing diffusion bonding or mechanical bonding between the anvil and the friction stir welding tool or work piece when friction stir welding high temperature materials or work pieces.
2. Background and Related Art
Friction welding has been used for years in the welding industry. For example, when the ends of two pipes are pressed together and simultaneously turned in opposite directions to one another heat is generated and causes the ends of the pipes to become plasticized and bonded. Rapidly stopping rotation of the pipes causes the two pipes to fuse together.
Friction stir welding, on the other hand, is a relatively new technique, and was first described in U.S. Pat. No. 5,460,317, issued Oct. 24, 1995. Friction stir welding involves pressing a non-consumable friction stir welding tool with a profiled probed end against two work pieces at the point where they come in contact with one another. Cyclical movement of the probed end of the tool generates heat as it presses against the two work pieces. The probed end of the tool enters or plunges into the two work pieces in a plasticized region created by the friction of the probed end. The probed end can then be slowly moved to a region where the two pieces abut each other, thereby enabling the area between the two pieces, which is beneath the probed end, to solidify together.
There are a number of advantages to friction stir welding. Among these include the fact that during friction stir welding the material heated is not exposed to combustion products. This reduces chemical changes in the work pieces due to the interface of the work pieces with the tool and its byproducts. Another advantage of friction stir welding is that the temperature of the work pieces, even in the heated region, tends not to be as high as the temperature resulting from conventional welding processes. This reduced temperature reduces oxidation of the work pieces due to ambient atmosphere, thereby reducing the need to provide an inert atmosphere at the weld location.
Friction stir welding has traditionally been limited to welding of low melting temperature materials such as aluminum alloy, copper alloys, lead and magnesium alloys because of the wear on the probed end. These materials are effectively joined using standard steel backing supports or anvils. However, recent advances in technology allow for the friction stir welding of harder, previously unweldable materials.
In order to join so-called xe2x80x98higher temperature materialsxe2x80x99 such as steels, stainless steels, nickel alloy or titanium alloys, it is necessary to employ increased temperatures and forces. These increased temperatures and forces present new problems. For instance, such high temperature materials can lead to diffusion or mechanical bonding of the work pieces to the anvil. Moreover, friction stir welding of high temperature work pieces may cause unwanted and added deformation of the anvil.
The current trend in the art when joining high temperature materials is to avoid full penetration with the probed end so as to reduce the probability that diffusion or mechanical bonding of the work pieces to the anvil occurs. Unfortunately, if the probed end penetrates only a small amount into the joint, only a portion of the joint is heated and joined, and the weld is not as strong as when the weld is fully penetrated through the joint region. In addition, the material is not fully plasticized in the entire welded seam, causing a portion of the apparently welded region to be in the form of a lap weld, which is a region where the material from one of the work pieces overlaps onto, but does not fully bond with, the material of adjacent work pieces. The weakness of such a weld is not always obvious or at least visible upon a precursory inspection.
Recent trends in the art attempt to achieve full penetration welds. A number of inventions deal with adding a chamfer or a groove to the anvil or the work pieces. (See U.S. Pat. Nos. 5,611,479 and 5,769,306.) Additionally, a number of patents focus on using a feedback control system to control the depth of the pin, thereby controlling the level of penetration. (See U.S. Pat. Nos. 6,168,066 and 6,173,880.)
U.S. Pat. No. 6,168,066 incorporates a ceramic sensor plate for sensing magnetic fields and eddies to control the depth of the magnetic friction stir welding tool in softer non-magnetic materials.
Accordingly, it would be an improvement in the art to augment or even replace the current techniques with other techniques.
It is an object of some embodiments of the present invention to provide a friction stir welding anvil that forms a diffusion barrier between the work pieces and the anvil when friction stir welding.
It is another object of some embodiments of the present invention is to provide an improved friction stir welding anvil that allows for the welding of materials and work pieces normally considered xe2x80x98rigidxe2x80x99 and difficult to weld with conventional friction stir welding anvils.
A further object of some embodiments of the present invention is to provide an improved friction stir welding anvil that allows for full penetration welding of materials currently difficult to friction stir weld with conventional friction stir welding anvils.
Yet another object of some embodiments of the present invention is to provide an improved friction stir welding anvil that resists the usual deformation attributable to the anvil when friction stir welding high temperature work pieces and materials.
An even further object of some embodiments of the present invention is to provide a friction stir welding anvil that prevents diffusion bonding between the work pieces and the anvil when friction stir welding.
Another object of some embodiments of the present invention is to provide a friction stir welding anvil that prevents mechanical bonding between the work pieces and the anvil when friction stir welding.
A further object of some embodiments of the present invention is to provide a friction stir welding anvil that is either coated with or manufactured from at least one chemically inert material that enables a diffusion barrier to form between the work pieces and the anvil.
Another object of some embodiments of the present invention is to provide a number of materials that can be used to coat an anvil or from which an anvil can be manufactured so as to enable superior performance than is available with current friction stir welding anvils.
Further objects, features, and advantages of the invention will become apparent to those skilled in the art from a consideration of the following detailed description taken in combination with the accompanying drawings.
In a preferred embodiment, a system is provided whereby a friction stir welding anvil is manufactured from a chemically inert material such as an Oxide, Nitride, Carbide or Silicate. In the embodiment, the anvil is a generally flat plate or rotating wheel that supports the work pieces while friction stir welding. Alternatively, instead of the anvil being comprised of the aforementioned materials, the chemically inert material is used to coat the anvil.
The following are alternatives for the method of application or manufacturing of the anvil using the above referenced materials. In one embodiment, the anvil is created by directly spray-coating the metallic anvil with the chemically inert material. Alternatively, the anvil is coated with the chemically inert material by chemical or vapor deposition. The anvil may also be coated with the chemically inert material through anodizing or may be coated with the chemically inert material through ion beam sintering. Further, the anvil may be a complete insert comprised of the chemically inert material or the anvil may be coated with a powdered form of the chemically inert material. Furthermore, the anvil may be coated with a powdered form of the chemically inert material by slurry. Alternatively, the anvil may be coated with the chemically inert material by creating and bonding the chemically inert material simultaneously by reaction or transformation.
In accordance with embodiments of the present invention, the anvil""s geometry or shape can vary in order to accomplish more specific objectives, so long as the anvil is at least partially coated or comprised of the chemically inert material. Additionally, any chemically inert coating, that includes a metal combined with Oxygen, Nitrogen, Carbon or Silicon as a second or third component to respectively form an Oxide, a Nitride, a Carbide or a Silicate improves performance of the anvil in accordance with embodiments of the present invention.
Moreover, any chemically inert coating that includes a chemically inert material, such as but not limited to diamonds, improves performance of the anvil in accordance with embodiments of the present invention.
The principles of embodiments of the present invention enable full penetration during friction stir welding of traditionally non-friction stir weldable materials, such as steels, titanium, and other high temperature materials because the tool does not stick to the anvil or work pieces and an enhanced weld is achieved.
These and other features and advantages of the present invention will be set forth or will become more fully apparent in the description that follows and in the appended claims. The features and advantages may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. Furthermore, the features and advantages of the invention may be learned by the practice of the invention or will be obvious from the description, as set forth hereinafter.